Surgical implement detector utilizing capacitive coupling

ABSTRACT

An apparatus for detecting a surgical implement in human or animal tissue comprises a battery powdered marker. The marker is secured to the surgical implement and positioned within a surgical wound. A detection means has an antenna disposed in close proximity of the tissue. Means are provided for capacitance coupling of both the marker and the antenna to the tissue. A field generating means generates an electromagnetic field having a predetermined frequency band within the tissue and a signal generating means generates a signal having a predetermined frequency band ranging from about 10 MHz to 1 GHz. The signal generated by the signal generating means causes the field generating means to generate the electromagnetic field providing the marker with signal identity.

This application is a continuation of application Ser. No. 698,187 filedMay 6, 1991, now U.S. Pat. No. 5,105,829, which in turn is acontinuation of application Ser. No. 437,178 filed Nov. 16, 1989,abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for detecting a markedsurgical implement such as a sponge, clamp, or a catheter within asurgical wound in human or animal tissue irrespective of its position ororientation therewithin.

2. Description of the Prior Art

During the course of a surgical operation it is frequently necessary forarticles, such as surgical sponges, qauzes, instruments, needles, andthe like, to be placed into a wound cavity. Notwithstanding rigorousprecautions attendant surgical procedures, such items are sometimesinadvertently lost during surgery and remain within the patient. Whenthis happens, the patient can encounter serious consequences, includingpain, infection, intestinal obstruction, and even death. The problem ofretained surgical implements has existed since the earliest days ofsurgery. Procedures conventionally employed to prevent post-surgicalimplement retention include a manual search of the wound by the surgeonprior to closure and a careful accounting for all materials inserted andremoved from the wound. The accounting function is customarily carriedout by the operating room staff, usually the circulating nurse. Despitethese precautionary measures the accidental retention of surgicalimplements continues to occur with disturbing regularity, even inprestigious institutions, and is regarded by surgeons as a majorunsolved problem.

At present, manual search and physical count remain the primary methodsused in detection of retained surgical implements. Most surgicalinstruments are composed of metal, and are easily detectable by x-ray.Sponges have been tagged with radiopaque markers to make them alsovisible on x-ray, but x-rays are not routinely done before completion ofthe operation, because of several disadvantages including inconvenience,expense, loss of operative time, and radiation exposure. Postoperativex-rays suffer from some of the same disadvantages. Moreover, even whenpostoperative x-rays are taken, retained items are occasionallyoverlooked; but even if detected, require a second operation to effecttheir removal.

To overcome the difficulty of detecting retained surgical implements, ithas been suggested that the implements be provided with a radioactivetracer. This technique, disclosed by U.S. Pat. No. 2,740,405 to Riordan,is subject to obvious hazards associated with use, storage and disposalof radioactive materials.

It has also been proposed that surgical sponges be marked with aflexible plastic impregnated with either paramagnetic or ferromagneticmaterials in the form of powders. Detection of these marked sponges isaccomplished by a metal detector. This method, taught by U.S. Pat. No.3,422,816 to Robinson et al., provides every small signals difficult todetect over the width of a patient's body. In addition, the Robinson etal. technique provides no discrimination against other metal objects,such as staples which, though present within the surgical wound, areappointed for retention therewithin.

Yet another proposal, advanced by U.S. Pat. No. 3,587,583 to Greenberg,involves use or surgical sponges marked with magnetized particles whosepresence is detectable with magnetodiodes. In practice, however, themagnetic field generated by these particles is too small to be readilydetected by the diodes.

U.S. Pat. No. 4,114,601 to Ables discloses the use of a smalltransponder fixed to a surgical sponge or instrument. This transponderexhibits gyromagnetic resonance at preselected frequencies. Detection isaccomplished by nonlinear mixing of two frequencies impinging upon thetransponder. The gyromagnetic resonance effect disclosed by Ables is ahigh frequency phenomenon, existing at frequencies of the order of about5 gigahertz (5,000,000,000 cycles/sec). These frequencies, known asmicrowaves, are absorbed readily by animal tissue and are, in fact, usedin microwave ovens for cooking. In use of the Ables type transponder,the energy developed goes primarily into heating tissue, rather thanexciting the transponder into gyromagnetic resonance.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for accurately andreliably detecting surgical implements within animal or human tissue.The apparatus comprises a detector responsive to the presence, within asurgical wound, of a surgical implement to which a marker is secured.The marker is adapted to produce, within the wound, identifying signalcharacteristics having a frequency band above 10MHz and below 1GHz.

Generally stated, the apparatus comprises a marker that is batterypowered. The marker is secured to a surgical implement positioned withina surgical wound. A detection means has an antenna disposed in closeproximity of said tissue. Means are provided for capacitive coupling ofboth of said marker and said antenna to said tissue. A field generatingmeans generates an electromagnetic field having a predeterminedfrequency band within said tissue; and a signal generating meansgenerates a signal having a predetermined frequency band containedwithin a range of about 10MHz to 1GHz. The signal generated by thesignal generating means is operative to cause the field generating meansto generate the electromagnetic field, providing the marker with signalidentity.

More specifically, the marker comprises a transponder enclosed within awater-tight case. A detector placed in very close proximity to thepatient is adapted, irrespective of the orientation of such marker, todetect the marker signal and record such detection with visual and/oraudio indicators.

Advantageously, the method and apparatus of the invention detectretention of surgical implements with far greater accuracy than methodsand means involving a physical count of implements that enter and exitthe wound. The apparatus is inexpensive to construct, safer for thepatient than postooperative X-rays and avoids risk to the environmentposed by radioactive tracers. Generation of a strong signal is effectedin a highly reliable manner. The signal is more easily distinguishedthan signals generated by magnetic detection systems, and is generatedwithout the heating of tissue caused by microwave detection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood and further advantages willbecome apparent when reference is made to the following detaileddescription of the preferred embodiment of the invention and theaccompanying drawings in which:

FIG. 1 is a schematic diagram depicting capacitance coupling betweenmarker and detector;

FIG. 2 is a block diagram of a surgical implement detector incorporatingthe present invention;

FIG. 3 is a block diagram of a marker;

FIG. 4 is a block diagram of a marker having an external tail antenna;

FIG. 5 is a digram of a marker having a reed-switch starting means;

FIG. 6 is a side-view diagram of a wax-spacer starting means;

FIG. 7 is an isometric view of a shrink-tube starting means; and

FIG. 8 is a schematic diagram of an electrical circuit for anonrechargeable marker.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method and apparatus of this invention make use of phenomenaoperative when an antenna is placed in very close proximity with thebody. Such placement of the antenna can, in certain cases, cause it tocapacitively couple to body tissue. In accordance with the invention, ithas been found that under certain criteria, a marker within the bodywill also capacitively couple with body tissue. This capacitancecoupling due to tissue 2 can be modeled as shown in FIG. 1, where theantennas of the marker 20 and detector 30 are coupled by capacitor 3 andresistor 4. Capacitor 3 is the equivalent capacitance between fieldgenerating means 27 of marker 20 and antenna 32 of detector 30. Resistor4 is the associated dc resistance of the signal path. The impedance tothe flow of signal, decreases with increasing frequency. At some lowfrequency, signal transfer due to tissue capacitance is insignificant.Further it has been found that no useful coupling occurs below 10Hz.This result is evidenced by TV(54-890MHz) and FM(88-108MHz) radioreception, when can be enhanced by proximately of the human body to thereceiver antenna, whereas AM(535-1605KHz) radio reception is notaffected.

This model for tissue capacitance does not hold as frequencies exceedabout 1GHZ; instead such frequencies are absorbed by tissue. Accordinglythe enhancement of signal transfer between marker antenna and detectorantenna due to tissue capacitance occurs solely at frequencies rangingfrom about 10MHz to about 1GHz.

Referring to the drawings, there is shown in FIG. 2 a block diagram of asurgical implement detector incorporating the present invention. Amarker 20 is secured to a surgical implement 16, such as a sponge,positioned adjacent tissue 2 within surgical wound 12. The marker 20 hasmeans for capacitance coupling with tissue 2 within would 12 of patient10. The marker 20, illustrated in FIG. 3, comprises a case 28 composedof a material, such as ABS plastic, that will remain inert within thewound for the period of the surgery. Within the case 28 is housed asignal generating means 22, a field generating means 27, a startingmeans 26, a power means 24, and a recharging means 25. The configurationof field generating means 27 depends on the frequency of the signalproduced by the signal generating means 22. The field generating means27 typically comprises an antenna 29 having a single lead, 2" or less inlength, would within the case and attached at one end to the signalgenerating means 22. Alternatively, as shown in FIG. 4, the antenna 29can be lengthened and connected through the case as a tail, thusincreasing the capacitance coupling with the surrounding tissue 2. Inthe embodiment of FIG. 4, the antenna 29, outside case 28, comprises aflexible, insulated wire.

Upon being activated by a signal from the detector, shown generally at30, the starting means 26 causes the power means 24 to be connected tothe signal generating means 22 which, in turn, is connected to the fieldgenerating means 27. The resulting field is transmitted for apredetermined period of time. The starting means 26 is comprised of anarrow band receiver 26a tuned to receive said signal from detector 30,a timing circuit 26b, and a switch 26c. Timing circuits are well knownin the art and can be constructed from a type 555 timer, where aresistor and capacitor establish said predetermined time. Switch 26c istypically a simple transistor. The power means 24 is typically arechargeable battery, such as NiCd, periodically refreshed by therecharging means 25. The recharging means 25 is comprised of a coil ofwire connected to a ac to dc rectifier. An external ac source isinductively coupled via a coil to the coil in the recharging means 25.Alternatively, the recharging means 25 is comprised of two externalcontacts through the case 28. Recharging is accomplished by directconnection with an external dc source.

The detector 30 is comprised of an antenna 32, an electronic switch 31,a controller 37, an interrogator 38 and its amplifier 35, a signalprocessor 36 and its filtering preamplifier 33, and an indicator 34. Thecontroller 37 acts as a common timing element for the processor 36, theinterrogator 38, and the switch 31. The switch 31 connects the antenna32 to either the processor preamp 33 or the interrogator amp 35 andcontains the appropriate impedance matching elements for eachconnection.

When activated by operating room personnel, the controller 37 causes theswitch 31 to connect antenna 32 to position 39a. The controller nextcauses the interrogator 38 to generate a single signal burst or,alternatively, a periodic signal burst, enhanced by amplifier 35 andtransmitted through switch 31 and antenna 32 to the marker 20, therebyactivating the marker's starting means 26 as described above. The signaltransmitted into tissue 4 by the marker 20 is received by the detector30 through its antenna 32, placed within close proximity of patient 10.The controller next causes switch 31 to connect antenna 32 to position39b. After filtering and amplification by preamp 33 and verification ofthe received signal by the processor 36, the indicator 34 is activated.The indicator is comprised of visual and/or aural transducer, such as alight and/or a buzzer. The configuration of the detector antenna 32depends on the frequency of the marker signal, as does the marker'sfield generating means 27. Antenna 32 is typically a single-end leadplaced under or next to the patient 10. Alternatively, antenna 32 is aconductive strap attached around patient's ankle, wrist, chest, pelvis,or thigh.

Alternatively, the power means 24 is not rechargeable but comprises analkaline or lithium battery. The signal transmitted by the marker 20 isof sufficiently long duration, typically at least 1/f milliseconds wheref is the frequency of transmission in kilohertz, to be detected by thedetector 30. In this manner, power usage is minimized and battery likeis maximized. The marker construction is simplified by elimination ofthe recharging means 25.

In a second embodiment of the invention, described with reference toFIG. 5, switch 31, interrogator 38 and amplifier 35 of detector 30 areeliminated. With this embodiment, the starting means 26 of marker 20must be manually activated by operating room personnel before the markedimplement 16 is placed within the wound 12. Marker 20 is rechargeableor, alternatively, nonrechargeable as described in the previousembodiment. The starting means 26 is comprised of a normally-closed reedswitch 42. A permanent magnet 44 placed on the outside of the casecauses the switch 42 to be in the open position. Removing,demagnetizing, or reorienting magnet 44 by 90 degrees causes the switch42 to close, thereby energizing the generating means 22. This, in turn,causes the marker 20 to transmit a periodic signal. Detector 30, withthe elimination of the interrogator 38, works as described in theprevious embodiment. Replacing, remagnetizing or reorientating themagnet 44 causes switch 42 to open and marker 20 to cease transmitting.

In a third embodiment of the invention, described with reference to FIG.6, the starting means can only be activated once, the activation beingaccomplished by operating room staff prior to the marker's first use.The marker's power means 24 is nonrechargeable and the recharging means25 is eliminated. The starting means 26 is heat-activated and itcomprises a spring-loaded battery contact 52 and an insulating waxspacer 54. Alternatively, battery 24 is coated with wax 54, therebybeing electrically insulated from battery contacts 52 and 56. The wax 54is selected to have a melting point preferably at the standard operatingpoint of gas sterilizers, approximately 140 degrees Fahrenheit. Beeswaxand paraffin meet this criterion. Melting of the wax 54 causes thespring-loaded or cantilevered contacts 52 and 56 to close againstbattery 24 energizing the generating means 22, whereby marker 20transmits a periodic signal. Alternatively, as shown in FIG. 7, startingmeans 26 is comprised of a sleeve of heat-shrink tubing 62 placed aroundtwo cantilevered contacts 63. Heat-shrink tubing typically requirestemperatures of 100 degrees centigrade to activate and cannot thereforebe in low temperature gas sterilizers but must be activated by steam orboiling water baths. Heating causes tubing 62 to contract and close thetwo cantilevered contacts 63, causing in turn marker 20 to transmit aperiodic signal. Detector 30 works as described in the previousembodiment.

To further illustrate the present invention, a marker was constructedusing the circuit shown in FIG. 8. Numerous other circuit configurationsare viable, but the circuit specified in FIG. 8 was chosen for itssimplicity. Inductors 76 and 78 were 1/4 inch diameter by approximately1/3 inch length air-core coils, each comprising 10 turns of 24 gaugemagnet wire. Resistor 74 was 200K ohms and capacitor 72 was a 10 mfdTantalum. NPN transistor 70 was 2N2222. Battery 24 was a 1.5V buttontype EP675. The circuit was placed on a 3/4 inch diameter mounting boardand placed within a water-tight case with a threaded lid. No antennalead was used. The marker emitted a broad frequency signal ranging fromabout 15MHz to 30MHz and modulated at approximately 1Hz. A marker wasplaced within a human cadaver. The detector comprised an ordinaryshortwave radio with an insulated wire monopole antenna placed on theoperating table along side the body. All orientations of the marker weredetected.

Having thus described the invention in rather full detail, it will beunderstood that such detail need not be strictly adhered to but thatvarious changes and modifications may suggest themselves to one skilledin the art, all falling within the scope of the invention as defined bythe subjoining claims.

What is claimed is:
 1. An apparatus for detecting a surgical implementin human or animal tissue, comprising:(a) a marker secured to a surgicalimplement adapted to be positioned adjacent tissue within a surgicalwound, said marker being battery powered and including a starting meanshaving a receiver means for receiving a first signal and a switchconnected to said battery for activating said marker; (b) detectionmeans having (i) an interrogator means for generating and transmittingsaid first signal to said starting means, (ii) a signal processor meansfor receiving a second signal and (iii) an antenna adapted to bedisposed in close proximity of said tissue, said antenna being a singleand lead end being alternately connected to said interrogator means andsaid signal processor means for generating said first signal andreceiving said second signal to detect said marker; (c) means forcapacitive coupling of said marker to said tissue; (d) means forcapacitive coupling of said antenna to said tissue; (e) field generatingmeans for transmitting an electromagnetic field having a predeterminedfrequency band within said tissue; and (f) signal generating meansactivated by said starting means in response to receipt of said firstsignal to cause said field generating means to transmit saidelectromagnetic field, providing said marker with signal identity, saidstarting means operating in response to said first signal to close saidswitch, connecting said signal generating means with said battery, saidelectromagnetic field being said second signal, said field generatingmeans being a single end lead, and said frequency band being containedwithin a range of 10 MHz to about 1 GHz.
 2. An apparatus as recited inclaim 1, wherein said detection means further includes an indicatingmeans for providing an audible or visible alarm when said marker isactivated.
 3. An apparatus as recited in claim 1, wherein said batteryof said marker is rechargeable.
 4. An apparatus as recited in claim 1,wherein said marker further comprises a recharging means
 5. An apparatusas recited in claim 4 wherein said recharging means of said marker isadapted to be inductively coupled to an external ac power source.
 6. Anapparatus as recited in claim 4, wherein said recharging means of saidmarker is adapted to be directly connected to an external dc powersource.
 7. A method for detecting a surgical implement in human oranimal tissue, comprising the steps of:(a) attaching a marker to asurgical implement appointed for disposition adjacent tissue with asurgical wound; the marker being battery powered; (b) disposing anantenna in close proximity of said tissue; (c) effecting capacitivecoupling between said marker and said tissue; (d) effecting capacitivecoupling between said antenna and said tissue; (e) transmitting anelectromagnetic field upon receipt of a first signal, saidelectromagnetic field having a predetermined frequency band within saidtissue; (f) generating said first signal to initiate transmission ofsaid electromagnetic field, said electromagnetic field being a secondsignal that provides said marker with signal identity, said frequencyband being within a range of about 10 MHz to about 1 GHz; and (g)receiving said second signal to detect said marker.